1. Field of the Invention
This invention relates to a light emitting module in which a light emitting element is mounted on a wiring substrate. In particular, it relates to a light emitting module which is used in a flat illuminating device.
2. Description of the Related Art
A liquid crystal display device has a feature that it is thin and light weight, and demand thereof is being heightened in personal computers and so on. Therefore, demand for the liquid crystal display devices has been increasing but, since a liquid crystal display panel itself does not emit light, outside light or a flat illuminating device (auxiliary illumination) becomes necessary in order to recognize a display content thereof visually. Also, in these years, based upon significant increase of the demand in mobile devices such as a portable telephone and PDA (Personal Digital Assistance), space saving due to reducing thickness and space and power saving due to battery operation have become important problems of the liquid crystal display device. On this account, space saving of the liquid crystal display device is realized by reduction of thickness of a lighting device, and power saving is accomplished by adopting a light emitting diode (LED).
Then, in the past, in order to realize the power saving of a flat illuminating device (back light, front light, etc.), the flat illuminating device is configured by combining a light emitting module using LED and a light guiding plate. FIGS. 1A and 1B are a cross sectional view and a plan view showing a conventional light emitting module. A light emitting module 1 is one in which a light emitting component 3 with built-in LED 2 was mounted on a wiring substrate 4.
The light emitting component 3, as shown in FIG. 1A, has two lead frames 5 and 6, and the LED 2 is mounted on a tip part of the one lead frame 5. The other lead frame 6 and the LED 2 are connected by a bonding wire 7. The LED 2 is sealed by a transparent mold resin 8, and except for a portion corresponding to a front surface of the LED 2, an outer surface of the transparent mold resin 8 is covered by opaque (e.g., white colored) mold resin 9. Further, on an under surface of the mold resin 9, disposed are a mounting side outside electrode 10 which is conducted to the lead frame 5 on which the LED 2 is mounted and non-mounting side outside electrode 11 which is conducted to the other lead frame 6.
On the other hand, the wiring substrate 4 is one in which a pair of lands 13 and 14 are made to face to each other on a surface of an insulation substrate 12, and two wiring lines 15 formed on the surface of the insulation substrate 12 are made to be connected to the respective lands 13 and 14. And, by soldering the mounting-side outside electrode 10 and the non-mounting side outside electrode 11 to the lands 13 and 14, respectively, the light emitting component 3 is mounted on the wiring substrate 4.
However, in the suchlike light emitting module 1, there is a problem that light emission luminance of the LED 2 comes down by heat generation (heat loss) from the LED 2. FIG. 2 is a view showing a relation between LED temperature (ambient temperature) and luminance, in a naked situation that the LED 2 is not covered by the mold resins 8 and 9. When the light emitting module 1 is activated, temperature of the LED 2 increases little by little due to heat generation of the LED 2, and as a result, as shown in FIG. 2, light emission luminance of the LED 2 decreases little by little.
Furthermore, when temperature of the light emitting component 3 increases, a transmission factor of the transparent mold resin 8 decreases, and thereby, there is a problem that, a ratio of light emerging from the front surface of the light emitting component 3 out of light emitted by the LED 2 decreases and light utilization ratio decreases. FIG. 3 is a view (a unit of luminance of a vertical axis is made to be the same as that of FIG. 2) showing a relation between LED temperature (ambient temperature) and luminance, in a situation that the LED 2 was covered by the mold resins 8 and 9. Comparing FIGS. 2 and 3, decrease of the luminance is more significant in the case of FIG. 3 where the LED 2 is covered by the resins 8 and 9 but, this is caused by such a matter that, when the transparent mold resin 8 is exposed to high temperature, it is deteriorated little by little (transparency falls down) so that a transmission factor thereof decreases.
On that account, in a conventional light emitting module shown in FIG. 4, by disposing heat dissipation means, temperature increase of the light emitting module is prevented. In this light emitting module 16, as shown in FIGS. 4A and 4B, a dummy wiring 17 is extended from the wiring line 15 of a side which is connected to the land 14, and a heat dissipation plate (heat sink) 19 made by a metal frame is soldered to, or secured by rivets to a land 18 which is disposed at a tip of the dummy wiring 17, and the heat sink 19 is folded toward an upper surface side of the wiring substrate 4 with a space so as not to become an obstacle.
By the suchlike heat dissipation means, heat generated in the light emitting component 3 is dissipated from the heat sink 19 through the dummy wiring 17 and the land 18. However, in the suchlike structure, there is a problem that size or thickness of the light emitting module 16 is enlarged because of the heat sink 19, and a space for disposing the light emitting module 16 is enlarged. Also, in the suchlike light emitting module 16, after the light emitting component 3 was mounted, the heat sink 19 has to be folded, and there is a problem that the number of man-hour for manufacturing the light emitting module 16 increases.